The X-ray luminosities in quiescent phase for superflare stars
Ken Yabuki1 , Yohko Tsuboi1 , Ryo Sasaki1 ,Yasuharu Sugawara2 , Satoru Katsuda1 , and Hiroyuki Maehara3
1

Department of Physics, Facility of Science and Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
2ISAS, JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 229-8510, Japan
3Okayama Astrophysical Observatory, National Astronomical Observatory of Japan,
3037-5 Honjo, Kamogata, Asakuchi, Okayama, Japan, 719-0232
E-mail(KY): yabuki@phys.chuo-u.ac.jp
Abstract
Maehara et al. (2012) and Shibayama et al. (2013) have searched for superflares on G-type dwarfs using
Kepler data in the optical band. They found that slowly rotating stars, with a spin period of about 20 days
like the Sun, caused superflares. It is believed that slowly rotating stars have a low X-ray luminosity anda small magnetic flux. Hence the superflares that occur on slowly rotating stars are strange phenomena.
Here, we have searched for Lxq of superflare stars, using available X-ray archival data. As the result, wefound that 11 stars are in the field of view of XMM-Newton and 7 of them are detected. In addition,
we confirmed that 3 stars are identified with sources in the ROSAT All-Sky Survey Faint Source Catalog(Voges et al. 2000). We discovered a positive correlation between Lxq and the maximum flare energy
during the Kepler observation (1.4 year) Etot,opt : Etot,opt ∝ L1.2
xq . In addition, there is no correlation
between Lxq with Prot for superflare stars. These results indicate that the flare mechanism is common
while the generation mechanism of the magnetic fields is diﬀerent.
Key words: stars:flares—stars:G type dwarfs

1.

Introduction

The X-ray emission from stars is associated with the stellar magnetic field. In fact, Pevtsov et al (2003) reported
that the quiescent X-ray luminosity are proportional tothe magnetic flux. In addition, the quiescent X-ray luminosity declines with increasing rotation periods (Prot )for the stars with Prot of more than 7 days. Hence slowly
rotating stars have a small magnetic flux. Since flares arecaused by releasing magnetic energy, it is believed thatslowly rotating stars cannot cause large flares.
However, the large flares occurring on slowly rotatingstars were reported by Maehara et al. (2012). Maehara
et al. (2012) searched for superflares on G-type dwarfs in
optical white light using Kepler data and found superflares on slowly rotating stars with a spin period of about
20 days like the Sun. They derived the total bolometric energy of the superflares and found that the largest
values of those for each object do not show any clear
correlations with the period of the stellar rotation. This
result indicates a diﬀerent trend from that obtained inX-rays. Therefore we investigate the correlation between
the maximum energy of white light flares during Kepler
observation (1.4 years)(Etot,opt ) and quiescent X-ray lu-minosity (Lxq ) for each object.
2. Analysis and result
Target stars are 279 superflare stars reported inShibayama et al. (2013). To find superflare stars detected in X-rays, we searched for X-ray archival data.
As a result, 11 stars were located in the field of view ofXMM-Newton and 7 stars were detected. 3 stars wereidentified with the sources in ROSAT All Sky Survey
faint Source catalog. One of the three stars was also
detected with XMM-Newton. Except for these stars, 4stars were in the field of view of Chandra, although nostars were detected. In total, we found 9 superflare stars
detected in X-rays. We investigate these stars in thiswork.
First, we made a light curve for each star to study
time variations. We fit each light curve with a constant.
As a result, the reduced χ2 is less than 2.0 for each star.
Therefore we consider that these stars are in the quiescent phase. Second, we calculated Lxq . For the stars
detected with significance levels larger than 7σ, we fit
the spectra with a thin thermal plasma model to calculate Lxq . For the other stars, we calculated Lxq by using

31Fig. 1. The relation between Etot,opt and Lxq . The vertical axis
is the maximum total flare energy in the observation period (1.4years) and the horizontal axis is Lxq .

webpimms assuming a thin thermal plasma model; we assume NH , kT and the abundance as 1018 cm−2 , 1.0 keV,and 0.4, respectively. Consequently, Lxq distributed at
the range of 1029 –1031 erg s−1 . Figure 1 shows the relation between Etot,opt and Lxq . We found the following
relation:
1.2+0.3

Etot,opt ∝ Lxq −0.4

(1)This equation implies that the maximum energy of whitelight flares is determined by the quiescent X-ray luminosity.
3. Discussion
3.1. Comparing with X-ray relation
In X-rays, Sasaki et al. (2017) investigated the relation
between the maximum energy of the X-ray flares (Etot,x )and Lxq using MAXI data. They found the following
relation:±0.04

Etot,x ∝ L1.26
xq(2)

In spite of the diﬀerent energy ranges, both Equation(1) and (2) are roughly proportional to Lxq , and the index of Lxq are the same within the error ranges. Thismeans that the ratio of the maximum flare energy between white light and X-rays is constant even though
the quiescent X-ray luminosity is diﬀerent.
The target stars of Sasaki et al. (2017) are several
types of stars (RS CVn systems, Algol systems, dMe,dKe, Young Stellar Object and K-type variable star). On
the other hand, our target stars are only G-type dwarfs.
Although target stars are diﬀerent, we obtained a similar

292827Log(X−ray Luminocity[erg/s])

30

3736
3534log(Total flare energy [erg])

282930log(Quiescent X−ray Luminosity [erg/s])

110Period[day]Fig. 2. The relation between Lxq and the rotational period. The
vertical axis is Lxq and the horizontal axis is the rotational period.
The stars and the circles indicate superflare stars and the G-typestars on which superflares have not been detected, respectively.

relation with Sasaki et al. (2017). This implies thatEquation (1) holds for several types of stars.
4. The diﬀerence of superflare stars and non-superflarestars
It is widely known that Lxq correlates with Prot among
G-type stars on which superflares have not been detected(we called these stars as non-superflare stars) (Pizzolatoet al. 2003). On the other hand, among superflare stars,
Lxq do not correlate with Prot (Figure 2).
In general, rapidly rotating stars generate strong magnetic fields. If the generation mechanism for the magnetic fields of superflare stars is the same as nonsuperflare stars, we can predict that superflare stars
have the same Lxq –Prot relation as non-superflare stars.
However, our result is inconsistent with this prediction.
Hence, we can consider that Figure 2 indicates the possibility that the generation mechanism for the magnetic
fields of superflare stars is diﬀerent from non-superflarestars.
References
Maehara, H., et al. 2012, Nature, 485, 478
Pevtsov, A., et al. 2003, ApJ, 598, 1387
Pizzolato, N., et al. 2003, A&A, 397, 147
Sasaki, R., et al. 2017, in these proceedings
Shibayama, T., et al. 2013, ApJS, 209, 5